The present invention relates generally to planar lightwave circuits. More particularly, the present invention relates to a method and system for a reduced polarization dependent wavelength shift/polarization dependent loss of planar lightwave circuit.
Planar lightwave circuits comprise fundamental building blocks for the newly emerging, modern fiberoptic communications infrastructure. Planar lightwave circuits are innovative devices configured to transmit light in a manner analogous to the transmission of electrical currents in printed circuit boards and integrated circuit devices. Examples include arrayed waveguide grating devices, integrated wavelength multiplexers/demultiplexers, optical switches, optical modulators, wavelength-independent optical couplers, and the like.
Planar lightwave circuits (PLCs) generally involve the provisioning of a series of embedded optical waveguides upon a semiconductor substrate (e.g., silicon), with the optical waveguides fabricated from one or more silica glass substrate layers, formed on an underlying semiconductor substrate. Fabrication techniques required for manufacturing PLCs using silica glass substrates is a newly emerging field. Electronic integrated circuit type (e.g., CMOS) semiconductor manufacturing techniques have been extensively developed to aggressively address the increasing need for integration in, for example, the computer industry. This technology base is currently being used to make PLCs. By using manufacturing techniques closely related to those employed for silicon integrated circuits, a variety of optical circuit elements can be placed and interconnected on the surface of a silicon wafer or similar substrate. This technology has only recently emerged and is advancing rapidly with leverage from more mature tools of the semiconductor-processing industry.
PLCs are constructed with a number of waveguides precisely fabricated and laid out across a silicon wafer. A conventional optical waveguide comprises an un-doped silica bottom clad layer, with at least one waveguide core formed thereon, and a cladding layer covering the waveguide core, wherein a certain amount of at least one dopant is added to both the waveguide core and the cladding layer so that the refractive index of the waveguide core is higher than that of the cladding layer. Fabrication of conventional optical waveguides involves the formation of a silica layer as the bottom clad (BC), usually grown by thermal oxidation upon a silicon semiconductor wafer. The core layer is a doped silica layer, which is deposited by either plasma-enhanced chemical vapor deposition (PECVD) or flame hydrolysis deposition (FHD). An annealing procedure then is applied to this core layer (heated above 1000 C.). The waveguide pattern is subsequently defined by photolithography on the core layer, and reactive ion etching (RIE) is used to remove the excess doped silica to form one or more waveguide cores. A top cladding layer is then formed through a subsequent deposition process. Finally, the wafer is cut into multiple PLC dies and packaged according to their particular applications.
A well-known problem with many PLCs is the polarization sensitivity of the device. Polarization sensitivity is a problem for both active PLC devices and passive PLC devices. For example, with arrayed waveguide grating (AWG) devices, integrated wavelength multiplexers/demultiplexers, and the like, due to the fact that an optical signal propagating through an optical fiber has an indeterminate polarization state, must be substantially polarization insensitive. However, due to stress imposed upon a silica substrates (e.g., from the fabrication process) and other factors, planar waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) propagation modes.
Polarization sensitivity is even more problematic with active PLC devices, particularly thermo-optic PLC devices where thermally induced birefringence exists in addition to any xe2x80x9cintrinsicxe2x80x9d birefringence of the PLC waveguides. PLC based thermo-optic devices utilize silica waveguides which exhibit the xe2x80x9cthermo-optic effectxe2x80x9d, wherein their refractive indices (e.g., the core refractive index) change as the temperature is changed. These thermo-optic effect based devices have been used in a variety of systems, such as, for example, optical switches, variable optical attenuators (VOAs), dynamic gain flattening filters (DGFF), and integrated devices such as a VMUX (VOA plus MUX) which require electrical circuits to activate the device.
Prior art FIG. 1 shows a diagram of a Mach-Zehnder thermo-optic switch. As depicted in FIG. 1, a first waveguide (core 10a) and a second waveguide (core 10b) are used to implement input ports (e.g., Pin1 and Pin2) and output ports (e.g., Pout1 and Pout2) as shown. The first and second waveguides pass through a first coupling region 21 and a second coupling region 22. A resistive thin film heater is deposited above each waveguide between the two coupling regions 21-22 (e.g., heater1 and heater2).
The heaters are used to selectively heat one waveguide core in order to change its refractive index, and thereby modulate an accumulated phase difference of light propagating through the two waveguide cores 10a-b. When light is launched into one of the input ports, it is split into the two cores 10a-b by the first coupler 21 with equal optical power and xcfx80/2 phase difference. As light travels through the waveguide cores 10a-b, the phase difference can be altered using a temperature difference between the two cores, as controlled by the two heaters. After passing through the second coupler 22, the two beams recombine either constructively or destructively at either of the two output ports, depending upon the exact phase difference between the two cores 10a-b. The exact phase difference is controlled by precisely controlling the current/voltage applied to the heaters. This modulation of temperature achieves the purpose of switching the light between the two output ports. The same technique can be used in VOA devices and other types of thermo-optic active PLC devices.
There exists a problem, however, in that the thermal stress-induced polarization sensitivity causes birefringence problems within the devices. As described above, the thermo-optic devices rely upon the selective heating of the silica waveguides to modulate the relative refractive index of the waveguides. However, this heat also induces stress (e.g., due to different coefficients of thermal expansion of the core, top clad, bottom clad, etc.) within the silica structure of the waveguides. Thus, waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) propagation modes, and the propagation constants vary with the application of power to the heaters (e.g., p1 and p2). This mismatch can cause a polarization dependent loss, wherein either the TE or TM mode is attenuated within the optical waveguide structures to a greater degree than the other, and other types of problems.
Prior art FIG. 2 shows a graph depicting polarization dependent wavelength shift for TE and TM propagation modes. As depicted in FIG. 2, a TE signal component and a TM signal component are graphed after having experienced phase dependent wavelength shift (PDW) due to, for example, thermally induced birefringence. The vertical axis of the graph shows insertion loss in decibels and the horizontal axis shows power (e.g., as applied to the heaters). The difference in propagation constants for the TE and TM signal components results in a PDW wavelength shift in the spectral response peak between the TE and TM signal components. As is known by those skilled in the art, this birefringence is characterized by the expression dn/dt.
Prior art attempts to solve the above described birefringence problem are described by Okuno et al. in xe2x80x9cBirefringence Control of Silica Waveguides on Si and Its Application to a Polarization-Beam Splitter/Switch,xe2x80x9d Journal of Lightwave Technology, Vol. 12, No. 4, April 1994, and by Yaffe et al. in xe2x80x9cPolarization-Independent Silica-on-Silicon Mach-Zehnder Interferometers,xe2x80x9d Journal of Lightwave Technology, Vol. 12, No. 1, January 1994. Okuno approaches waveguide birefringence by depositing one or more amorphous silicon patches on top of the Mach-Zehnder arms of a device and by laser trimming. Yaffe approaches waveguide birefringence by adding SiN4 patches under the Mach-Zehnder arms of a device. In both prior art cases, the intrinsic birefringence of the silica waveguide structure is controlled, not the thermally induced birefringence generated due to phase shifter heater temperature, as in thermo-optic PLC devices. Also, in both prior art cases, additional film deposition steps are required in the fabrication process flow.
Thus what is required is a solution that matches the TE and TM propagation modes of an optical signal within active PLC devices. What is required is a solution that minimizes thermally induced dn/dt birefringence within thermo-optic PLC devices. The present invention provides a novel solution to the above requirements.
The present invention is a method and system for reducing dn/dt birefringence in a thermo-optic PLC device. The present invention provides a solution that matches the TE and TM propagation modes of an optical signal within active PLC devices. The present invention minimizes thermally induced dn/dt birefringence within thermo-optic PLC devices. Additionally, the present invention does not add additional film deposition steps to the PLC device fabrication process.
In one embodiment, the present invention is implemented as a PLC device fabrication process for making optical waveguide structures for thermo-optic PLC devices having a reduced dn/dt birefringence. The process includes the step of forming a waveguide core layer on a bottom cladding, the waveguide core layer having a higher refractive index than the bottom cladding. The waveguide core layer is then etched to define a waveguide core. A top cladding is subsequently formed over the waveguide core and the bottom cladding. The top cladding also has a lower refractive index than the waveguide core. The top cladding is then etched to define a first trench and a second trench parallel to the waveguide core. The first trench and the second trench are configured to relieve a stress on the waveguide core. This stress can be induced by a heater, as in a case where the active PLC is a thermo-optic PLC.
Depending upon the specific requirements of a device, the first trench and the second trench can extend from the upper surface of the top cladding to the upper surface of the bottom cladding, or deeper. For example, in a Mach-Zehnder switch application, the first trench and the second trench can extend from the upper surface of the top cladding into a portion of the underlying substrate.
The first trench and the second trench are configured to balance a stress within the waveguide core. Both tensile stresses and compressive stresses exist within the waveguide structure due to the thermal expansion characteristics of the top and bottom cladding and the waveguide core. The first and second trenches balance the stresses by allowing the top cladding to more readily expand. A cap can be formed over the waveguide core prior to forming the top cladding to further balance the stress within the waveguide core. Additionally, the bottom cladding can have a higher dopant concentration than the top cladding to further balance the tensile stress within the waveguide core.